1. Field of the Invention
The invention relates to the use of a monovalent ligand of the FcαRI IgA receptor as an anti-inflammatory agent.
2. Description of the Background
Immunoglobulin A (IgA) is the most heterogeneous Ig isotype in humans, existing in multiple molecular forms such as monomeric, polymeric and secretory IgA; it comprises two subclasses IgA1 and IgA2.
In serum, IgA exists mainly in monomeric form, with a minor percentage of polymeric IgA (pIgA).
In mucosal secretions (saliva, tears, colostrum, gastrointestinal fluids, nasal bronchial secretion, and urine), IgA is produced as dimers, joined by a polypeptide termed J-chain. Dimeric IgA binds to the membrane-associated polymeric Ig receptor (pIgR), and the resulting complex is transported from the baso-lateral to the apical/luminal side of mucosal epithelium. During this transport the bound IgA is released by proteolytic cleavage from the pIgR; however a portion of the pIgR, the secretory component, remains associated with dimeric IgA, forming altogether secretory IgA (SIgA).
SIgA plays a major role in the innate immune system preventing microorganisms and foreign proteins from penetrating the mucosal surfaces. It also neutralizes toxins and infectious organisms.
Whereas the role of secretory IgA is established in mucosal immunology, the function of serum IgA antibodies is mostly unknown. Although IgA is the second most abundant Ig isotype in serum, it is not usually involved in humoral immune responses and does not activate complement. Monomeric serum IgA has anti-inflammatory activity and is capable of down-regulating functions such as IgG-induced phagocytosis, bactericidal activity, oxidative burst, and cytokine release. In contrast, polymeric IgA and IgA-containing immune complexes (IC) can efficiently trigger immune effector functions on blood leukocytes through IgA Fc receptors.
Receptors for the Fc region of immunoglobulins (FcRs) play a major part in the link between humoral and cellular responses. FcRs for all five human antibody classes have been described.
The human IgA Fc receptors (FcαR) family comprises several members (for review cf. MONTEIRO and VAN DE WINKEL, Annu. Rev. Immunol. 21: 177-204, 2003), but only FcαRI (or CD89), a receptor specific for the IgA Fc region, has been identified on blood myeloid cells (MONTEIRO and al., J. Exp. Med. 171: 597-613, 1990; MALISZEWSKI and al., J. Exp. Med. 172: 1665-1672, 1990). FcαRI is expressed on monocyte/macrophages, dendritic cells, Kupffer cells, neutrophils and eosiniphils and binds both IgA1 and IgA2 (CONLEY and DELACROIX, Ann. Int. Med. 106: 892-899, 1987; KERR, Annu. Rev. Immunol. 12: 63-84, 1994) with low affinity (Ka≈106 M−1) (MONTEIRO and VAN DE WINKEL, 2003, aforementioned).
FcαRI is a member of the Ig gene superfamily. It comprises two extracellular Ig-like domains (EC1 and EC2), a transmembrane region and a cytoplasmic tail devoid of recognized signaling motifs. Crystal structures of human FcαRI reveal that the two Ig-like domains are oriented at right angles to each other and that two FcαRI molecules are required for the binding of one IgA molecule (HERR and al., J. Mol. Biol. 327: 645-657, 2003). The IgA binding site is located in the membrane-distal EC1 domain. Anti-FcαRI mouse and human monoclonal antibodies (mAb) have been generated (MONTEIRO and al., J. Immunol. 148: 1764-1770, 1992; SHEN et al., J. Immunol. 143, 4117-4122, 1989; PCT WO 91/05805; PCT WO 02/064634), and it has been shown that monoclonal antibodies that bind in the EC1 domain of FcαRI block IgA binding, whereas those that bind in EC2 do not.
Due to the moderately fast on- and off-rates of the FcαRI:IgA binding reaction, monomeric IgA binding is transient, whereas polymeric IgA and IgA immune complexes bind with a respectively growing avidity due to a decrease in the off-rate (HERR and al., 2003, aforementioned; WINES, J. Immunol. 162: 2146-2153, 1999).
The involvement of FcαRI in the ability of IgA to trigger immune responses such as phagocytosis, antibody-dependent cell-mediated cytotoxicity (ADCC), superoxide generation, cytokine production, antigen presentation and inflammatory mediator release, has been reported (for review, see MONTEIRO and VAN DE WINKEL, 2003, aforementioned). It has been proposed to use anti-FcαRI antibodies, such as My 43 (PCT WO 91/05805), or the human monoclonal antibodies disclosed in PCT WO 02/064634, to activate these FcαRI-mediated immune responses.
It has also been proposed to use anti-FcR antibodies, including anti-FcαRI antibodies, as vectors for selectively targeting active principles, such as cytotoxic compounds, to cells expressing Fc receptors (PCT WO 99/41285).
U.S. Pat. No. 6,018,031 describes bifunctional antibodies containing the binding region of an anti-FcαR antibody and the binding region of an antibody directed against a target cell. These bifunctional antibodies can bind on one hand said target cell, and on the other hand effector cells expressing FcαR. Their binding to FcαR triggers the FcαR-mediated activity of the effector cell, resulting in the destruction of the target cell bound to the same bifunctional antibody molecule.
Signaling through FcαRI is dependent on association of FcαRI with the FcRγ chain subunit, forming the trimer FcαRIα/γγ. The FcRγ chain contains an immunoreceptor tyrosine-based activation motif (ITAM) in its cytoplasmic tail (PFEFFERKORN and YEAMAN, J. Immunol. 153: 3228-3236, 1994; LAUNAY and al., J. Biol. Chem. 274: 7216-7225, 1999) that allows the recruitment of crucial signalling effectors (KINET, Annu. Rev. Immunol. 17: 931-972, 1999). FcαRI can be expressed with or without physical association with FcRγ subunit. The γ-less FcαRI internalises and recycles IgA to the cell surface, whereas FcRγ-associated FcαI directs complexed IgA to lysosomes (LAUNAY and al., 1999, aforementioned; SHEN and al., Blood 97: 205-213, 2001). No cellular function of non aggregated FcαRI, other than IgA recycling, has so far been identified. Receptor aggregation is required for FcαRI-mediated activation of target cell functions such as cytokine release and antigen presentation (SHEN and al., 2001, aforementioned; PATRY and al., Immunol. 86: 1-5, 1995; GEISSMANN and al., J. Immunol. 166: 346-352, 2001).
While involvement of FcαRI in IgA-mediated inflammation is well recognized, the molecular basis that underlies the IgA anti-inflammatory capacity has not been elucidated until now. Although it has been reported (WILTON, Clin. Exp. Immunol. 34, 423-8 1978; VAN EPPS and WILLIAMS, J Exp Med 144, 1227-42 1976) that IgA inhibitory functions require the Fcα region, the part played by IgA Fc receptors remains unknown.
A consensus model of negative signaling in the immune system involves receptors with an immunoreceptor tyrosine-based inhibitory motif (ITIM) in their cytoplasmic domain. These inhibitory receptors act by co-aggregating with activatory receptors: cross-talk between the two receptors generates a negative signal (RAVETCH and LANIER, Science, 290, 84-89, 2000). An example of the ITIM class of inhibitory receptors is the Fcγ receptor FcγRIIB. However no ITIM receptor for the Fcα region is known.